Method of depassifying high chromium steels prior to nitriding



5. LOW

Sept. 9, I958 METHOD OF DEPASSIFYING HIGH CHROMIUM STEELS PRIOR TONITRIDING Filed ma a, 1957 United States Patent METHOD OF DEPASSIFYINGHIGH CHROMIUM STEELS PRIOR TO NITRIDING Sidney Low, Wilbraham, Mass.,assignor to The Chapman Valve Manufacturing Company, Indian Orchard,Mass., a corporation Application May 8, 1957, Serial No. 657,863 3Claims. (Cl. 14816.6)

My invention relates to new and useful improvements in the surfacehardening of ferrous alloys and is directed particularly to an improvedprocess for nitriding chromium-containing ferrous alloys including inparticular the austenitic high chromium and high chromium-nickelstainless steels and rustless irons and to the resultant nitrided steelproducts and manufactures.

The products hereof offer improved qualities of high wear resistance,especially for applications where high surface hardness (even afterheating at elevated temperatures of as high as 1100 F.), high resistanceto corrosion, low tendency to seize or gall, minimum warpage ordistortion, and improved fatigue resistance, are desiderata.

Hardening processes of the known prior art have comprised the heating ofthe work piece in contact with ammonia gas, usually at elevatedtemperatures in the range of 800l100 F. for a predetermined period oftime which may vary from as short as a few minutes to as great as onehundred or more hours, depending upon the specific results desired ineach particular instance.

During this period of time, nitrogen, which is liberated by thedecomposition of the ammonia, is absorbed by and forms nitrides with theiron and the alloying elements, such as aluminum, chromium, molybdenum,nickel, vanadium, and/ or other nitride-hardening elements, present inthe steel.

The nitrides of the alloying elements are precipitated at the nitridingtemperature along the crystal planes of the iron resulting in theproduction of a hard, wearresistant case.

As is well known, the nitrides, which are in a fine state of dispersionin the case, impart an extreme hardness to the surface of the steel, thechanges associated with the formation of the alloy nitrides at thenitriding temperature accounting for the hardened case and the hardeninggradually decreasing inwardly until it corresponds to that of the core.

At the lower temperatures of the range, between 800- 900 F. a very hardcase results but little penetration is offered so that the use of thesetemperatures is unsuited for valves, valve seats and the like, which aresubjected to wear or more or less localized stresses.

For any given temperature within the range, the penetration may beincreased by increasing the duration of the nitriding reaction but theincrease of penetration is not commensurate with the increased time, i.e., the rate of penetration falls off rapidly after 15 to 20 hours.

As is also 'well known, the nitriding process has enjoyed widecommercial use heretofore but diificulties have been encountered inapplying it to the high chromiumnickel ferrous alloy compositions.Unless these alloys are given a special preliminary treatment, a caseeither is not formed at all within any reasonable time of treatment oris too shallow or of too low a degree of hardness to accomplish thedesired result.

Ferrous alloy articles or work pieces to be hardened are usuallymachined to final shape and dimensions before being subjected to thenitriding treatment.

The chief difliculty in nitriding the high chromium ferrous alloys hasbeen attributed to the presence on the surface of the alloy of an inertor passive film composed of chromium oxide. A characteristic quality ofthis surface oxide film is that it has a tendency to prevent the etfectsof the conventional nitriding treatment on the metal.

These alloys may be made more susceptible to nitriding by a depassifyingtreatment involving etching of the surfaces of the article to benitrided with a hydrogenliberating acid such as hydrochloric acid. Whilesuch treatment will accomplish the desired purpose, it involves theintroduction of an additional step, not to mention the expense, in thepreparation of the articles for nitriding. Too, it necessitates theintroduction of the work piece into the nitriding furnace immediatelyfollowing the depassifying treatment. Failure so to do will result inthe oxide film re-forming upon exposure to air. Obviously, unlessspecial precautions are taken, the results obtained with the use of suchan acid are not uniformly dependable.

An electrolytic cleaning operation followed by a washing stepimmediately preceding the introduction of the work piece into thenitriding furnace has also been employed successfully. Here again,however, extra operations and increased costs are involved.

High chromium ferrous alloys have also been successfully nitrided withan initial reduction of the passive oxide layer by a treatment withhydrogen in the same furnace in which the nitriding is carried out. Suchprocedure has been found to give a satisfactory nitriding result, butnecessitates additional equipment for cracking ammonia so as to furnishthe hydrogen for the reduction of the oxide film.

In some prior art practices, it has been found necessary and/ordesirable to roughen the surface of the work piece as by sandblasting orthe like prior to the actual nitriding operation in order to achieveoptimum nitriding results. For the obvious reason that such a practicedestroys the oftentimes painstaking and careful machine work representedin the work piece, not to mention the sometimes spoilage of work wheretolerances are an especially critical consideration, the procedure isundesirable. Furthermore, unless the initial cleaning procedure isfollowed with reasonable promptness by the nitriding operation, an oxidefilm forms anew on the metal. Too, the very shape or size of the workpiece has made it difiicult or impossible to subject the metalsatisfactorily to the essential cleaning operation. Where sandblastinghas been employed, a certain roughness is imparted to the metal, whichroughness is frequently undesirable and objectionable as aforesaid. Samemay not only spoil the appearance of the finished hardened product butalso affect the utility thereof. The sandblasting or like treatment,when brought to bear upon small or delicate objects, might even resultin obviously objectionable deformation or distortion.

A still further objection to the known and conventional nitridingprocedures resides in the frequent obtainment of nitrided surfaces whichlack uniformity of hardness and wear-resistance. This non-uniformityapparently is due to an inefficient or improper removal of the passiveoxide film initially or is due to a reformation of the oxide subsequentto the cleaning operation and prior to the nitriding.

In this invention, I provide an economical and industrially practicalprocess of nitriding stainless steel products wherein the cleaning andsurface hardening are achieved in a thoroughly satisfactory manner witha minimum of operational steps.

It is a principal object hereof to provide a method of nitriding alloysteel, especially stainless steel, which as a procedure is characterizedby directness and simplicity, and ensures the obtainment of uniformlynitrided steel surfaces at greatly increased rates.

It is another object hereof to provide a nitriding process applicable tothe various high-chromium ferrous alloys, including those of austeniticstructure and in particular the stainless steels of the high-chromiumand austenitic high chrome-nickel types, which is substantially freefrom the objections of the above cited prior art processes.

It is a further object to provide a thoroughly practical and highlyeffective method of removing the outermost oxide film from the metaldirectly as a part of the nitriding procedure and of rapidly andefliciently casehardening the same in a nitrogenous atmosphere.

A further object hereof is to provide a nitriding process by which highchromium ferrous alloys may be satisfactorily nitrided withsubstantially no sacrifice of time and with only a moderate increase ofcost as compared with the conventional nitriding process as it isapplied to the loW-chromium-aluminum nitriding steels.

Another object hereof is to provide a nitriding process applicable tohigh chromium ferrous alloys by which a nitride case of markedlyincreased depth can be obtained in a given nitriding cycle as comparedwith the results obtained by following any of the above referred toprior practices.

It is a still further object hereof to provide a durable, wear-resistantcase-hardened chromium ferrous alloy product which individuallypossesses high surface hardness, which hardness moreover is uniformalong the surface.

The nitriding treatment alone, by way of an atmosphere of a nitrogenliberating gas, fails to serve effectively to remove the oxide film fromthe metal so as to ensure the desirable formation of a uniformly hardnitrided case, resistant to wear and corrosion.

I introduce outside of or to the nitriding retort a substantial amountof a fluorocarbon so as to make available therein an abundance of gasupon the decomposition thereof. Said gas is directed to the vicinity ofthe steel and accomplishes the desired oxide film removal.

Subsequently, I introduce a nitrogenous atmosphere comprising suchamounts of ammonia gas as to make available an abundance of freenitrogen in the vicinity of the steel for the completion of thenitriding process.

I have determined that the above itemized purposes and ohiects can beattained and high chromium ferrous alloys can be provided with a nitridecase of good characteristic in a conventional nitriding furnace andwithin the normal range of nitriding temperatures, provided thepreparation for nitriding is carried out in the presence of one of agroup of fiuorocarbons in close contact with the work piece surface tobe nitrided.

The materials which have been found to give this beneficial result arethose which have the capacity, upon being heated to predeterminedelevated temperatures, of emitting gaseous derivatives of fluorine.

Preferably, the fluorocarbon employed is one whose range ofdecomposition is such that it become active at approximately thetemperature at which the nitriding operation is carried out.

These gaseous derivatives etch the surface of the work piece within theretort just prior to the nitriding action. The etching action offers animproved or super active surface resulting in an accelerated nitridingrate.

The heart or real spirit of the invention resides in the sequentialsteps of making the surface of the work piece active while within thenitriding retort and nitriding immediately there-following withoutinterruption.

A surface, so prepared, is thus activated so as to be more receptive tothe nitrogen within the retort than has heretofore been possible toachieve. Such activation constitutes the process of reducing the film ofany oxide material upon the surface practically as an integral part ofthe nitriding process.

With this method, the work pieces may be prepared for the treatment inany of the conventional ways and it is unnecessary to introduce anyspecial operation for depassifying the work piece surface so as toprepare same for the nitriding.

The invention consists in the combination of elements, composition ofmaterials, and mixture of ingredients, and in the several operationalsteps, and in the relation of each of the same to one or more of theothers, as described herein.

The invention, broadly defined, generally relates to the process ofnitriding steels at an accelerated rate by means of a fluorine gascapable of activating the surfaces thereof so as to make same morereceptive to the nitrogenous gas, same constituting a sequential step inthe treating process. The invention further includes the nitridedarticles of manufacture produced by said process.

As conducive to a clearer understanding of certain features of theinvention, it may be noted at this point that stainless steel is definedas a low-carbon alloy metal which comprises from about to approximatelychromium, with or without nickel, and with or without the supplementaladdition of manganese, silicon, cobalt, copper, molybdenum, tungsten,vanadium, columbium, titanium, and/ or sulphur, and a balance ofsubstantially all iron.

The process of this invention is normally applied to steels of theaustenitic stainless type although conceivably it can be used for theferritic or martensitic steels as well.

Austenitic stainless steels, where the chromium is the predominatingalloy addition to iron, varying from 16 to 26% in content, are generallyenvisioned for the purposes hereof. Particular reference will be madeherein, by way of illustration only, to A. S. T. M. A276 Type 304 withan 0.08% carbon maximum in the popular 18 chromium-8 nickel steel. Asanother example, reference will also be made herein to A. S. T. M. A276Type 410 stainless steel.

Austenitic steels have desirable characteristics in that they resistcorrosion but the surface characteristics thereof are not such that wearis resisted and they cannot be hardened as may other steels. When partsmade from austenitic steel are in contact with one another, or when apart made from austenitic steel is in contact with a part made fromanother steel, and when there is any relative movement of those parts,the austenitic steel has a tendency to scratch, score, seize and/orgall. For instance, when a valve or a seat of a valve is made fromaustenitic steel, the action of one part on another causes theaustenitic steel to scratch, score, seize and/or gall so that the unitis rendered unfit for service, in spite of the fact that the steel stillhas retained its desirable ability to resist corrosion.

In this invention, austenitic steel is treated in such a Way that it ishardened, or at least its surface is hardened, to the extent that it isdefinitely wear-resisting and does not tend to scratch, score, seize, orgall, while its resistance to corrosion and temperature is in no wayimpaired.

By means of this invention, I provide a method of treating steel of theaustenitic class, and the ferritic and martensitic classes as well,whereby the treated steel has an improved hardened Wear-resistingsurface and also has new corrosion and temperature resistingcharacteristics. Such a steel is especially adapted for use under themore unfavorable conditions of elevated temperature and/or associationwith agents capable of bringing about eXCCSsiVe oxidation and corrosion.

Further, by means hereof, I provide an article of manufacture having anitrided surface properly supported by an cates with the conduit 22.

underlying core capable of withstanding extreme operating stress withoutsuch plastic deformation as will cause a cracking or failure otherwiseof the hard nitrided case.

Usually the steel work piece is heated in a nitriding furnace subjectedfirst to the action of a fluorine gas and then to an atmosphere of anitrogenous medium such as ammonia gas under conditions whereby surfacehardness is imparted to the material by the absorption of the nitrogen.

In the accompanying drawing, I have illustrated a complete example of aphysical embodiment of the invention in which the steps are combined andarranged in accordance with one mode which I have devised for thepractical application of the principles of the invention. It will beunderstood however that changes and alterations are contemplated and maybe made in the procedures hereof and in the exemplifying drawing, allwithin the scope of the claims, and without departing from theprinciples of the invention.

In the drawing, the figure is a diagrammatic elevational view of anapparatus adapted for carrying out the novel method of the invention.

Referring now to the drawing more in detail, the novel apparatus andmethod of the invention will be fully described.

In this disclosure, I show, for illustration purposes, a completeexample of a physical embodiment of the invention in which the steps arecombined and arranged in accordance with one mode which I have devisedfor the practical application of the principles of the invention.

Means for supplying ammonia gas may include a supply source or tank suchas and the gas is conducted therefrom through a suitable conduit 12having a valve means 14 and a pressure indicating means 16 providedtherealong.

The conduit 12 connects directly to one side of a dissociation furnace20, on the opposite side of which, a conduit 22 having a valve 23therein leads away therefrom to a nitriding retort 50 which is disposedin a suitable electric, gas or oil fired batch-type metallurgicalfurnace 52.

A by-pass conduit 24, having one or more suitable valves 26 therealongmay be provided to by-pass the furnace 20, if desired.

Another conduit 30 may be provided and communicates at one extremitywith the conduit 12 and at its opposite extremity with a decomposingchamber 32. A valve 34 may be provided in the conduit 30.

On the opposite side of the chamber 32 and communicating therewith, aconduit 36 may be provided.

The opposite extremity of the conduit 36 communi- A valve member 38 maybe provided in said conduit 36. A heating element 33 may be provided inconjunction with the decomposing chamber 32.

Additionally if desired, another conduit 40 connecting with conduit 30and a decomposing chamber 42 substantially as shown, may be provided. Avalve 44 may be provided in the conduit 40. On the opposite side of thedecomposing chamber 42 and communicating therewith, a conduit 46 may beprovided.

The opposite extremity of the conduit 46 communicates with the conduit36, substantially as shown. A valve 48 may be provided in conjunctionwith the conduit 46. A heating element 43 may be provided in conjunctionwith the decomposing chamber 42.

The nitriding retort 50 receives the work piece bein treated, and isheated by a heating element 53. r

A water bath 60 may be provided including a water inlet 62 and a'wateroutlet 64. Inlet 62 and/ or outlet 64 may be provided with a valve means66 if desired.

Leading from the retort 50 is a conduit 70 through which the exhaustproducts of the nitriding action may pass from the retort to the bath 60for absorption therein.

The conduit 70 may be provided with a pressure indicating means 72, avalve means 74, and an indicating means 76 to show the volume (bypercentage) of the dissociation of the ammonia gas in the retort 50.

The mechanical elements having been described, one method of applyingthe principles of the invention will not be defined in the followingessential series of sequential steps.

' For a. continuous supply of the treating ammonia gas to the nitridingretort, I prefer to employ a tank of commercial anhydrous ammonia(represented by the source of supply 10) from which gaseous ammonia isconducted under a predetermined positive pressure through conduit 12 tothe dissociation furnace 20 whereat the hydrogen and nitrogenconstituents of the ammonia gas are dissociated. The control ismaintained over the gas flow through the use of a pressure regulatingvalve 14.

The dissociation furnace 20 is heated to a temperature within the rangeof l200-1600 F.

The decomposing chambers 32 and 42 may be employed if it is desired todecompose the fluorocarbon in an area other than with the nitridingretort.

Further, it is conceivable that it may be desired under certaincircumstances to supplement the flow of the dissociated ammonia gas asit leaves the dissociation furnace 20 with the gaseous derivatives ofthe fluorocarbon from the decomposing chambers 32 and/or 42 so as toproceed through conduit 22 to the nitriding retort 50 simultaneously.

Conceivably, the fluorocarbon can be decomposed in the decomposingchambers 32 and/ or 42 or in the nitriding retort 50, all as may bedesired The work piece W to be treated is placed in the nitriding retort50 and the quantity of polytetrafluoroethylene generally indicated by Fin the drawing is placed either in the decomposing chambers 32 and/or 42or in the nitriding retort 5 0.

The nitriding retort, after having been purged with NH to removeatmosphere therefrom, is heated to a temperature of 900 F. Which ismaintained for a period of sixty minutes. Simultaneously therewith, thedissociationfurnace is heated to 1300 F. Specific temperatures areindicated as preferred.

The entire piping system, dissociation furnace, nitriding, retort, etc.are purged with NI-I before heating. The dissociation furnace is thenheated to 1200l600 F. with a small flow of NH flowing thru the system.When the dissociation furnace reaches operating temperature, the NH,flow is adjusted to a flow of dissociated ammonia of 4 to 50 C. F./H.The nitriding retort is then heated to 950-1120 F. The fluorocarbon inthe nitriding retort begins to decompose at approximately 500 F. Therate of decomposition is proportional to the temperature. At 500 F., thefluorocarbon decomposes slowly, at 900 F., rapidly. During the period ofdecomposition of the fluorocarbon; the nitriding retort is filled withan atmosphere of dissociated NI-I plus gaseous decomposition products ofthe fluorocarbon. This atmosphere reduces the oxide film on the worksurface and prepares it for nitriding.

When the fluorocarbon is completely decomposed (usually within theheating up period of the loaded nitriding retort) the dissociationfurnace is shut off, valve 23 closed, valves 26 opened and the NH flowadjusted so that the dissociation measured by instrument 76 is 5 to 20%(by volume).

The flow of NH is maintained at this quantity for a period of 2 to 20hours, depending on the case depth desired. A nitriding period of 15hours will produce a case depth of approximately 5 /2 mils on theaustenitic stainless steels and a case depth of approximately 11 /2 milson the ferritic or martensitic stainless steels.

Under the influence of the heat within the retort, the C F begins todecompose so that the work piece W is subjected to the action of. thedecomposing fluorocarbon,

the gas circulating around and on all sides of the work piece W.

The materials falling Within the fluorocarbon classification which Ihave tested and found suitable for use in the present process aretetrafluoroethylene, trifluorochloroethylene resin, Freon and otherfluorocarbons.

Each of these resins, when heated above 750 F., decompose slowly toyield a gaseous monomer together with small amounts of other gaseousderivatives of fluorine.

There is a wide latitude in the choice of the temperatures and timesemployed. The choice of these factors generally depends upon the retort,the time available, the metallurgical effects on the base metal, andrelated factors.

When the work piece W has thus been heated and subjected to the etchingaction of the gas of the fluorocarbon, it is then subjected to theaction of the products of the ammonia gas. That is, the heated steel isput in contact with a source of active nitrogen at the specifiedtemperature for periods ranging from a few minutes to 100 hoursdepending upon the steel being treated and the depth of case desired.

The ammonia gas is conducted to within the nitriding retort by means ofthe conduit 22. The gas circulates around and on all sides of thestainless steel work piece Wand thence from the nitriding retort throughgas outlet conduit 70 to suitable disposed means 60.

As treatment of the work pieces continues at nitriding temperatures inthe presence of the fluorine gas, the oxide film is effectively removedfrom the surface of the article and due to the action of the nitrogenousgas, a hard nitrided core begins to form.

Over an extended period of continued treatment, the nitrided layergradually increases to a useful thickness; usually a treatment periodranging from approximately 30 to 90 hours gives a thickness which issatisfactory for most articles, although I do not Wish to be bound bysuch period.

The time of treatment will vary according to the size and shape of thesteel, and to the particular results desired. The time treatment mayvary from a few minutes to as much as 100 hours.

The rate of flow of the ammonia is adjusted so that the percentagedissociation of the exhaust gases is held at between substantially15-50%.

The process of the present invention is equally applicable with otherammonia dissociation values. It may be employed, for example, inconjunction with the process disclosed in the patent of Carl F. Floe,#2,437,249, dated March 9, 1948, in which process the ammoniadissociation value is held relatively low in the earlier stage of thenitriding operation and thereafter is raised.

The heat is maintained at the stated temperature for a predeterminedtime following which the flow of the ammonia gas products is cut off.

An excess fluorine in the retort reacts with the ammonia gas to form astable salt which is disposed of as a waste product.

As examples of the process employing C 1 (polytetrafluoroethylene), testspecimens of both A. S. T. M. A276 Type 304 stainless steel and A. S. T.M. A276 Type 410 stainless steel placed in the nitriding furnace alongwith pieces of the fluorocarbon. The retort was heated to 900 F. and thefluorocarbon was decomposed.

The ammonia gas, with a dissociation ranging from 5 to 20% wascirculated into the retort.

As to the general metallography of the test specimens, they were foundto have representative cases and interface structures for theirrespective material types. The case was found to be uniform in hardnessand case depth for the section viewed. Photomicrographs, etched andunetched at 100x, for both materials, were prepared. The case depths,Rockwell 15 N surface hardness, and hardness traverse follow.

A. l. S. I. Type 304Sample #1343 Knoop Hardness Values 500 Gram LoadCase Depth-Inches Cracked 1, 058 756 A. I. S. 1. Type 410Sample #1344Knoop Hardness Values 500 Gram Load Case Depth-Iuches Cracked 8 (Uponremoval from the furnace and after cooling, the test specimen was foundto have a hardness of 91-92 on the Rockwell 15 N scale, as contrastedwith its initial hardness of 77 on the same scale. Transverse sectionsof this specimen were prepared and examined microscopically and werefound to have a nitride case of good characteristics extending to adepth of 0.0045).

It will be appreciated that the invention is not limited to theparticular nitriding procedures disclosed herein. The disclosure is madeby way of illustration and not of limitation. Those skilled in the artwill also appreciate that the process is applicable tochromium-containing ferrous alloys of other compositions than thosementioned by way of example above including, among others, variousstainless steels, rustless irons and high chromium cast irons, whereinvarious other alloying additions are present besides chromium orchromium and nickel. The duration of the treatment also may be changedin accordance with the requirements of the particular work to beprocessed.

Having thus described the invention and the best method of practisingthe new process for preparing this steel without being limited to theorder of steps of such process as herein recited, or to the proportionsof parts employed therein, or to the precise ingredients named therein,it being evident that each of these steps has a considerable range ofequivalents, and it being also evident that the order and proportions ofthe process may be carried out without departing from the scope andpurposes hereof, what it is desired to claim and secure by LettersPatent of the United States is:

1. In a method of conditioning chromium steel for a nitriding treatment,the art which includes, heating the steel within a sufficiently hightemperature range in the presence of the gaseous derivatives of afluorocarbon and the removal of the chromium oxide film on the steelsurface, and further heating the steel within a sufficiently hightemperature range and for a sufficiently long period of time in anitrogenous atmosphere.

2. The improvement in the nitride hardening of a high chromium ferrousalloy characterized by the presence on the surface thereof of a passiveoxide film serving to inhibit penetration of nitrogen into said surfaceportions when such articles are heated in a nitrogenous atmospbere,comprising, heating the ferrous article directly and without apreliminary depassifying treatment at a nitriding temperature and in thepresence of the gas of a decomposing fluorocarbon, and continuing theheating treatment in the presence of a nitrogenous atmosphere.

3. The improvement in the nitride hardening of a high chromium ferrousalloy characterized by the presence on the surface thereof of a passiveoxide film acting to inhibit penetration of nitrogen into said surfaceportions when such articles are heated in a nitrogenous atmosphere,comprising, heating the ferrous article directly and without apreliminary depassifying treatment at a temperature between 900 and 1100F. initially in the 10' presence of the gases of a decomposingfluorocarbon and subsequently in the presence of a nitrogen-liberatinggas.

References Cited in the file of this patent 5 UNITED STATES PATENTS1,085,768 Thompson et a1. Feb. 3, 1914 1,958,575 Hengstenberg May 15,1934 10 FOREIGN PATENTS 366,838 Great Britain Feb. 11, 1932

1. IN A METHOD OF CONDITIONING CHROMIUM STEEL FOR A NITRIDING TREATMENT,THE ART WHICH INCLUDES, HEATING THE STEEL WITHIN A SUFFICIENTLY HIGHTEMPERATURE RANGE IN THE PRESSURE OF THE GASEOUS DERIVATIVES OF AFLOUROCARBON AND THE REMOVAL OF THE CHROMIUM OXIDE FILM ON THE STEELSURFACE, AND FURTHER HEATING THE STEEL WITHIN A SUFFICIENTLY HIGHTEMPERATURE RANGE AND FOR A SUFFICIENTLY LONG PERIOD OF TIME IN ANITOGEOUS ATMOSPHERE.